The present invention relates to methods and apparatus for machining one or more highly concentric diametral surfaces on a workpiece. However, certain applications of the present invention may lie outside this field.
It is conventional on some machining operations to machine a workpiece after applying compression to the workpiece. Often this compression force within the workpiece is applied through thrust bearings that are oriented so as to apply compression in the workpiece, but incapable of applying tension in the workpiece without degrading the reliability and safety of the thrust bearing. The compressive force tends to cause the workpiece to bow. Also, there is a natural tendency for the force from the cutting, grinding, milling or other operation, which is applied generally perpendicularly to the compressive force, to further increase the bowing of the workpiece. This bowing results in uneven machining of the workpiece, causing varying amounts of eccentricity among the one or more diameters machined along the length of the workpiece. For example, a diameter machined on a shaft may have excessive runout of the center of the machined diameter relative to the centerline of the shaft.
On workpieces where multiple outer diameters are being machined, the multiple outer diameters as first machined may not be sufficiently concentric with each other, or with the machining centers on the ends of the workpiece that may be used in subsequent operations. For example, two diameters may each be acceptably round, but their centerlines may be shifted from one another. In another example, one diameter may be acceptably round, but another diameter on the same workpiece may be out of round. It may be necessary to support the workpiece in the vicinity of the multiple outer diameters by a pair of rolls. It is often necessary to add a second machining operation to improve the relative concentricities of the multiple diameters. The present invention overcomes the limitations of conventional machining methods and apparatus in a novel and unobvious way.